Natural Law Theory

Here is a small video I created explaining Natural Law Theory (Below is transcript):

In this video I explore the strengths and weaknesses of Natural Law:

Transcript of Natural Law Theory

St Thomas Aquinas developed Natural Law theory from Aristotle and the Stoics in the 13th century. Natural Law is an absolutist and deontological theory. To believe and use Natural Law theory, one has to believe in God because Natural Law believes there is one Natural Law which has been issued by God. This means that what is wrong in one situation is wrong in ever situation and to determine what is right and wrong we look at the action itself, not the consequences. There is emphasis on innate human reason to work out how you should be living. Natural law is useful on issues where is the Bible is silent e.g. IVF.In order to use Natural Law correctly we need to identify what is known as the primary precepts. These are the basis of the theory. We need to first agree that these are the main function of life. The primary precepts are the five purposes of human life. The five primary precept are: (i) reproduction (ii) life – to live/ the supreme good (iii) education – makes people independent and fully adult (iv) worshipping God and (v) law and order – upholds justice. These precepts are immutable; they cannot be changed. From the primary precepts, secondary precept form. For example if we examine the principle of life and it is there to ensure that all actions which are life threatening are in the moral law as wrong e.g. murder, abortion, euthanasia, suicide etc are all wrong because the involve going against the precept of life.However, in reality it is not as easy as one action is good and one is evil. Some actions lead to both good and bad consequences which ever way you try and deal with it. For example, a pregnant women finds out she has cervical cancer. If the doctor performs a hysterectomy the baby dies and the mother survives. On the other hand, if she gives birth both the mother and the baby are at risk of dying. So whatever action the doctor takes there will both goos and bad consequences. This is called the principle of double effect. In these situations, two criteria need to be met to make it a permissible action. Firstly, the act itself must not be wrong and secondly, the person doing the action must not directly intend the evil outcome. So he doctor would do the hysterectomy with good intentions of saving the mother. This justifies his action.Casuistry is what applies these primary precepts to individual circumstances. These makes Natural Law quite flexible. It ensures that a solution can be produced from any situation. However, many consider this to be a weakness of the theory.They use the term casuistry pejoratively. This is because on one hand it states it is an absolutist theory and on the other it is allowing room for flexibility.

Natural Law allows us to know the divine and moral law through reason and revelation. Following Natural Law helps us perfect our virtues; both natural and theological. Natural virtues include prudence, temperance, fortitude and justice. Theological virtues include faith, hope and charity. For example, the primary precept of life means that life is extremely important. Therefore, the natural virtue of fortitude is perfected because no matter what happens, one is encouraged to stand strong and take care of our life and as mentioned before suicide will be considered immoral. 

Natural Law is based on five assumptions: (i) everything in nature has a purpose, (ii) nature was created by God, (iii) failure to develop this nature to the fullest is an imperfection, (iv) nature and its moral laws are knowable through reason and (v) natural law is part of some divine plan. Thats why for some people the theory fails. However, it has worked well and influenced the Roman Catholic denomination. For example, Roman Catholics believe abortion is wrong based on that it destroys the purpose of life: to live.

As mentioned, at the beginning Aristotleʼs philosophy influenced the Natural Law theory quite a lot. Aristotle in his theory of causality distinguishes between four causes which helps to explains something fully. Two of the four causes, efficient and final cause, Aquinas used to formulate the Natural Law theory. The efficient cause is what gets things done and the final cause is the end product. Hence, Aquinas goes on to say with us the accomplishment of the final cause/end product that equates to ʻgoodʼ we are doing the correct thing. If we can understand the final cause of an organism we can then work out the efficient cause/ the necessary processes to get the final cause done.

Natural Law is part of the branch of normative ethics. This is different to descriptive ethics. Natural Law is part of normative ethics because the impact of Aristotle’s causes moves it from being a theory that deals with what the population actually think it is right and wrong (descriptive ethics) to what they should think is right and wrong (normative ethics). This is because the natural processes required to fulfill the final cause are what should be right, not necessarily what is right at any given time. 

Root of Natural Law

Roots of Natural Law

  • The roots of Natural Law can be dated back to the Greek (8th – 6th C BC) and Roman era (27th BC to 476 AD).
  • However, the two man roots of Aquinas’ Natural Law lie in Aristotelian work and the Stoics. 


The Stoics existed in the 3rd C and they emphasized the importance of ‘logos’ (rationality that governs the world and sees human nature as part of one natural order). They believed that Natural Law is the Law formed by right reason. Right reason only serves its own ends and is not corrupted into serving special interests. This right reason helps humans to judge whether or not they want to follow a divine spark which existed in them. Being Pantheist they said humans have a divine spark within them which right reason helps them to follow.


Aristotle believed that we can judge the extent to which an action is good by seeing if it helps us to attain our ultimate good. It is difficult to see what our ultimate good is, it is not like maths. Higher social activities which other animals cannot do are ones which should be fulfilled in order to live a well functioning life. He also believed reason was key in working out ethics, this he called practical ethics. In ethics, Aristotle claimed, we go from ‘true but obscure judgements’ to general ethical principles (see parallel with primary and secondary principles). We can work out these principles by comparing to what extent they help us to achieve our ultimate good. 

In summary: Everything has a purpose and by fulfilling it we can reach our supreme good – eudaimonia.

Aquinas came along and identified three crucial things from Aristotle and the Stoics which he married with ideas of Christian Theology to create Natural Law theory. Those three points were:

  1. Human beings have an essential rational nature given by God in order for us to live and flourish.
  2. Even without knowledge of God, reason can discover the laws that lead to human flourishing.
  3. The Natural Laws are universal and unchangeable and should be used to judge the laws of particular societies.

Aquinas believed our telos isn’t eudaimonia buy rather perfection. We should strive for this because we are made in the image and likeness of God. Furthermore, for Aquinas this isn’t something which can be achieved in this life but rather starting in this life and continuing through to  the next.

Natural Law helps us to fulfil our telos of perfection and this in itself will help to live a fulfilled life and flourish.

Aquinas said that humans have a natural inclination to do good and avoid evil. Hence, he distinguished between real and apparent good. As no human would for this reason knowingly commit an evil action, he said people are evil unknowingly. They are following “apparent good” (something which seems to be good or the right thing to do but does not fit with the perfect human ideal). A “real good” on the other hand is the right thing to do, it fits the human ideal.